Noble metal-refractory metal alloys as catalysts and method for making

ABSTRACT

Finely divided noble metal-refractory metal alloys and methods for making them are disclosed. As catalysts these alloys have greater activity than a catalyst of the same unalloyed noble metal and may be advantageously used as electrodes for fuel cells particularly when supported. The method for making supported catalysts involves a simple and inexpensive procedure for converting a supported, finely divided noble metal catalyst to the desired alloy. In a preferred embodiment the process includes intimately contacting the supported noble metal catalyst with a finely divided refractory metal oxide, the metallic component of which is capable of enhancing the activity of the catalyst when alloyed therewith, and then heating to a sufficiently high temperature, preferably in a reducing atmosphere, to reduce the oxide and simultaneously form a finely divided, supported alloy of the noble metal and the metallic component of the oxide.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the production of supported noble metalalloy catalysts for use in fuel cell electrodes and other catalyticstructures.

2. Description of the Prior Art

A fuel cell is a device which converts the energy of a chemical reactionbetween a fuel and oxidant directly into low voltage, direct currentelectricity. The problems encountered in obtaining an efficient fuelcell are essentially those of chemical kinetics. It is necessary thatthe reactions of the fuel and oxidant occur in such manner that theamount of energy degraded into heat is as small as possible. At the sametime, the reaction rate of the cell must be high enough to produceeconomically sufficient current from a cell of practical size. For thisreason it is customary to incorporate catalysts in the fuel cell whichaccelerate the reactions occurring at the electrodes.

A typical fuel cell comprises a fuel electrode or anode, an oxidantelectrode or cathode, an electrolyte positioned between the electrodesand means to introduce fuel and oxidant to their respective electrodes.Each electrode is typically comprised of a substrate (such as wetproofedpaper made from graphitized carbon fibers) with a layer of catalystdisposed on the surface which faces the electrolyte.

In operation, the fuel (commonly hydrogen) is fed to the anode where itis oxidized at the catalytic surface in the presence of electrolyte withthe liberation of electrons. Simultaneously oxygen (or air) is fed tothe cathode where it is reduced at a catalytic surface in the presenceof electrolyte with the consumption of electrons. The electronsgenerated at the anode are conducted to the cathode through wiresexternal to the cell and constitute a flow of electrical current whichcan be made to perform useful work.

To obtain fuel cells which are practical on a commercial scale, a greatdeal of research has been carried out in an effort to find improvedcatalysts. For example, the prior art has demonstrated that the activityper unit mass of a catalyst, usually a noble metal, can be enhanced bysupporting it in the form of finely divided particles, upon eithermetallic or carbonaceous base materials of high surface area. Thisapproach has proved especially useful in fuel cell applicationsutilizing acid electrolytes, for example, where particulate platinum ishighly dispersed on a conductive support material such as carbon blackand the platinum-covered carbon black, mixed with a suitable bondingagent, is applied as a thin layer on a conductive carbon paper or clothto form an electrode.

In addition, the prior art has demonstrated that certain unsupportednoble metal alloy catalysts exhibit increased catalytic activity and/orincreased resistance to sintering and dissolution in fuel cells andother electrochemical and chemical processes when compared to theperformance of the unalloyed noble metal catalyst. For example, U.S.Pat. No. 3,506,494 describes a method for producing a ternary alloy foruse at the anode of a fuel cell. The ternary alloy consists of platinum,ruthenium and a metal selected from the following: gold, rhenium,tantalum, tungsten, molybdenum, silver, rhodium, osmium, or iridium.Although it states in column 3, at lines 67-70, that the alloy catalystsmay be dispersed on a high surface area carrier such as carbon powder,no method is taught for doing this.

U.S. Pat. No. 3,428,490 describes another method for making a fuel cellanode electrode. In this case unsupported platinum is alloyed withaluminum and applied to an electrode substrate. The aluminum is thenleached out to the extent possible to form the finished electrode. Theremoval of the aluminum produces a large number of reaction sites orvoids in the electrode. It is stated that the voids increase the surfacearea and thus the activity of the catalyst. Although this patentindicates in column 6 at lines 26-29 that some aluminum may still bepresent in the electrode composition after leaching, it is believed thatthe amount remaining is not significant and it would be present only inthose areas which could not be reached by the leaching solution. Thepatent teaches no method for making a noble metal-aluminum alloy whichis supported.

Patents of more general interest which relate to platinum alloycatalysts are U.S. Pat. Nos. 3,340,097 (platinum-tin-ruthenium) and3,615,836.

It is known that some alloys may be made by co-reducing intimatemixtures of reduceable metal salts. For example, the method ofco-reducing metal salts in the presence of a support material is used tomake a supported, finely divided platinum-iron alloy as explained in anarticle by C. Bartholomew and M. Boudart titled "Preparation of a WellDispersed Platinum-Iron Alloy on Carbon" from the Journal of Catalysis,pp. 173-176, V25, #1, April 1972. However, salts of certain metals arenot readily reduced. Such metals are those which form refractory metaloxides, e.g., Ti, Ce, Mg, Al, Si, and Ca.

It is apparent from the foregoing that there is still no commerciallyviable process for preparing high surface area catalysts of noble metalsalloyed with the metallic component of a refractory metal oxide. It hasbeen observed, however, that platinum and other noble metals and noblemetal alloys, in bulk form, react with many refractory metal oxides athigh temperatures to form solid solution alloys or intermetallic alloycompounds and that these reactions are accelerated by the presence ofreducing agents in the high temperature environment of, for example,carbon, hydrogen, carbon monoxide and certain organic vapors. "PlatinumMetals Review 20, " No. 3, p. 79, July 1976.

Finally, returning to the subject of fuel cells, all base metals,including the refractory metals, are notoriously susceptible tooxidation and dissolution at cathodes in acid fuel cells, and it is notbelieved that alloys of noble metals with base metals have ever beenconsidered for use at cathodes for that very reason, whether supportedor unsupported.

As used herein, "noble metals" refers to those metals of the second andthird triads of Group VIII of the Periodic Table, also referred to asthe palladium and platinum groups, respectively. These metals areruthenium, rhodium, palladium and osmium, iridium and platinum.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a process for preparingfinely divided alloys of a noble metal and a metal which forms arefractory oxide.

Another object of the present invention is a supported finely dividednoble metal alloy catalyst having high catalytic activity.

In the following discussion of this invention and in the appendedclaims, when catalytic activity comparisons are made they are intendedto be comparisons of mass activity. Mass activity is an arbitrarilydefined measure of the effectiveness of a catalyst per unit weight ofthe catalytic material. In the case of fuel cells with phosphoric acidas electrolyte we define the mass activity of the cathode catalyst (inmA/mg) as the maximum current available due to oxygen reduction at 0.900volts, the potential being measured relative to an unpolarized H₂ /Ptreference electrode at the same temperature and pressure in the sameelectrolyte. A greater mass activity can be achieved by eitherincreasing the surface area of the catalyst (e.g., by reducing theparticle size) or by increasing its specific acitivity. Specificactivity is defined as the O₂ reduction current, as specified above,which is available per unit surface area of the noble metal (i.e.,μA/cm²). The greater mass activity of the alloy of the present invention(in comparison to the mass activity of the unalloyed noble metal) isattained through improvement in the specific activity of the catalyticmaterial in comparison to the specific activity of the unalloyed noblemetal.

As used throughout the specification and claims, refractory metal isdefined as any metal which forms a refractory metal oxide.

In the process of the present invention finely divided unalloyed noblemetal particles and finely divided particles of an oxide of a refractorymetal are reacted to reduce the oxide and at the same time form a finelydivided alloy of the noble metal and refractory metal.

Preferably the reaction is effected by intimately contacting supportedfinely divided noble metal catalyst particles with fine particles of arefractory metal oxide and heating to a sufficiently high temperature(preferably in a reducing environment) to thermocatalytically reduce themetal oxide and simultaneously form an alloy containing the noble metaland the refractory metal. The novel resulting product is a supportedfinely divided alloy of a noble metal and refractory metal.Surprisingly, when used as a catalyst in a fuel cell cathode electrodeit has significantly enhanced overall catalytic activity relative toboth the supported unalloyed noble metal and the same alloy unsupported.Although during the process there is generally a loss in the supported,finely divided, unalloyed noble metal surface area due to thermalsintering, this loss in surface area is more than compensated by theincreased specific activity of the resulting catalyst. Notwithstandingthe foregoing, the surface area of the supported alloys of the presentinvention are still considered high relative to correspondingunsupported alloys.

The method is equally well suited to making finely divided unsupportedas well as supported alloys. However, since finely divided unsupportednoble metals are limited, generally, to less than 50 m² /g of noblemetal, this method is best practiced by using supported finely dividednoble metals, which can be prepared in surface areas generally greaterthan 100 m² /g of noble metal. It is preferred that the surface areas ofthe alloys of the present invention be greater than 30 m² /g of noblemetal in the alloy, and most preferably greater than 50 m² /g.

This product finds application not only in fuel cell electrodes but alsoin the chemical, pharmaceutical, automotive and anti-pollution fields.It may also have non-catalytic applications. By proper selection of thenoble metal and the base metal the supported alloy can be tailored tosuit particular service conditions.

The word "alloy" as used above and hereinafter in the specification andclaims is intended to encompass within its meaning solid solutions andintermetallic compounds of the metals being combined.

Commonly owned, copending U.S. Patent Application Ser. No. 922,003titled "Electrochemical Cell Electrodes Incorporating Noble Metal-BaseMetal Alloy Catalysts" by V. Jalan, D. Landsman, and J. Lee, filed oneven date herewith claims electrochemical cell electrodes incorporatingthe herein described novel alloy as a catalyst.

Commonly owned copending U.S. Application Ser. No. 922,005 titled "NobleMetal/Vanadium Alloy Catalyst And Method For Making" by V. Jalan, filedon even date herewith described a finely divided noble metal-vanadiumalloy and method for making said alloy.

The foregoing and other advantages and objects of the present inventionwill become more fully apparent from the following description ofpreferred embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with a preferred embodiment of the present invention,supported unalloyed noble metal catalysts in the form of finely-dividedparticles dispersed uniformly over the surface of a suitable supportmaterial are utilized. This form of the catalyst provides a high surfacearea, high activity catalytic structure, especially when the supportmaterial also is characterized by high surface area. (For some electrodeapplications and some non-catalyst applications a finely dividedunsupported noble metal alloy may be desirable. In that case thestarting material would be finely divided, unsupported, unalloyed noblemetal.) It is to be understood that "unalloyed noble metal catalyst"includes not only the noble metals discussed hereinbefore in elementalform but also mixtures or alloys of one noble metal with another. Othermetals not in the second and third triads of Group VIII of the PeriodicTable are excluded. In general, the support material provides bulk tothe catalyst making it practical to fabricate structures containing verysmall quantities of the noble metal while allowing attainment of muchhigher surface areas (and thus higher catalytic activity per unit massof noble metal) than are achievable with the unsupported noble metal, orare achievable with an unsupported noble metal alloy.

A variety of materials (e.g., silica and alumina) are used as supportsfor noble metal catalysts in industry. The usual criterion for selectinga material for use as a support is that it be chemically stable in theenvironment in which the catalyst operates. For electrochemicalapplications the support material should also be electrically conductiveand materials such as carbon, graphite or metals are commonly used. Thecharacteristics required of a successful support material are well knownor can be easily determined by those skilled in the art for a particularchemical or electrochemical service application.

An example of a preferred supported unalloyed noble metal catalyst ofthe type described above which has found use in electrodes for fuelcells employing a phosphoric acid electrolyte is a conductive carbonblack such as Vulcan XC-72 (made by Cabot Corp. of Billerica, Mass.)having finely-divided particles of platinum uniformly dispersed over itssurface. Techniques for providing the uniform dispersion of finelydivided platinum particles on the carbon black and other supportmaterials are well known in the art and are not considered to be part ofthe novel aspects of the present invention. Carbon is a preferredsupport material for many fuel cell applications. Some types of carbonsother than carbon black which may be used are graphite, partiallygraphitized carbon, and acetylene black.

The method of the invention is particularly advantageous since it nowprovides means for significantly enhancing the catalytic activity of asupported unalloyed noble metal catalyst. The invention is furtheradvantageous since the specificity, resistance to sintering, electronicand other physical properties of the catalyst can be tailored toparticular service applications by the proper selection of the noblemetal and refractory metal.

In accordance with a preferred embodiment of the present invention,supported unalloyed noble metal catalyst in finely divided form (i.e.,submicron high surface area) is reacted with a finely divided refractorymetal oxide, the metallic component of which is capable of significantlyenhancing the catalytic activity and/or other physicochemical propertiesof the noble metal when alloyed therewith. The first step in effectingthe reaction is to provide intimate contact between the metal oxideparticles and the supported noble metal catalyst particles. Although avariety of contacting techniques may be utilized, those found especiallyuseful involve soaking the supported catalyst in a colloidal dispersionof the metal oxide or soaking the supported catalyst in a solution of ametal compound which can be subsequently reacted or converted to thedesired metal oxide by simple and well known chemical processes such asthe thermal decomposition of the carbonate, sulfate or hydroxide or byhydrolysis of some other salt. The former technique may be used with ametal oxide which is readily available in finely divided form. Thelatter technique may be used when the metal oxide is not readilyavailable in finely divided form. After the oxide-impregnated catalystis dried, the product is an intimate mixture of fine noble metalparticles and fine metal oxide particles both supported on the supportmaterial of the original unalloyed noble metal. Further details of thesetechniques are provided in the examples set forth below for purposes ofillustration.

The intimate mixture of supported noble metal particles and refractorymetal oxide is then heated to a sufficiently high temperature that themetal oxide is reduced by the combination of temperature and proximityto the noble metal particles, whereby the metallic component of themetal oxide forms a solid solution or intermetallic compound with thenoble metal (i.e., an alloy). Typically in the invention, the intimatemixture must be heated to at least about 600° C., preferably 800°-1000°C., to achieve the desired reaction. Of course, the time at temperaturemust be sufficient to complete the reaction and will vary with the metaloxide chosen and the amount used, times of 1-16 hours generally beingsatisfactory in this regard. The heating is usually conducted in areducing atmosphere to accelerate the reaction, but an inert atmosphereor vacuum may be used in certain instances where a reducing environmentis present. For example, with a platinum-covered carbonaceous supportmaterial and metal oxide, the following reaction can occur to acceleratethe reaction:

    Pt/C+M(Ox)→Pt.M/C+CO.sub.2 ↑

where M represents a metal. Of course, a small but relatively harmlessamount of the carbon support material, which provides the locallyreducing environment, is consumed in this situation. In addition, somethermal sintering of the noble metal usually occurs during the heattreatment, but results have indicated that the loss is relativelyinsubstantial when the significantly increased specific activity orimproved performance of the resulting alloy catalyst is considered.

Some refractory metal oxides may not be available in finely divided formand there may be no presently known method for making these finelydivided oxides. This does not mean, however, that if the finely dividedoxide were available it would not work. On the contrary, theoreticallyany refractory metal (or any non-noble metal) may be alloyed with anoble metal by the disclosed process. The refractory metals which wehave already alloyed with platinum by the present method and used at thecathode of a phosphoric acid fuel cell are tungsten, aluminum, titanium,silicon, aluminum-silicon, cerium, and strontium-titanium. We have notas yet found any which do not work well as a fuel cell cathode catalystprovided the finely divided form of the refractory metal oxide can beobtained.

In the foregoing description the precursor noble metal is a supportednoble metal and one step in the process involves putting the metal oxideparticles on the support with the noble metal particles. For the purposeof the present invention it does not actually matter how these two typesof particles arrive on the support material. For example, unsupportednoble metal particles and metal oxide particles could be co-depositedonto the support material. It is important that both types of particlesbe finely divided and uniformly dispersed over the surface of thesupport. Preferably the size of the oxide particles should be about thesame as that of the noble metal particles. If the oxide particles aretoo large or are poorly dispersed the particles of the noble metal maysuffer excessive sintering during heating by coalescing with each otherinstead of reacting with the oxide particles. This could result in anunacceptable loss in catalytic surface area. For the same reasontemperatures in excess of 1000° C. should be avoided. In the context ofthe present invention, finely divided particles are particles ofsubmicron size.

The most effective amount of refractory metal in the alloys of thepresent invention will vary depending upon the application to which thecatalyst is to be put and the materials making up the alloy. As littleas one percent and perhaps even less may provide a noticeable increasein cathode catalytic activity. The best proportion can be determinedthrough experimentation. The maximum amount of refractory metal isdetermined by the solubility limits of the refractory metal in the noblemetal.

The following examples are offered to illustrate the process of theinvention in more detail, especially as it relates to preparing finelydivided supported noble metal alloy catalysts for use in acid fuel cellelectrodes:

EXAMPLE 1--Pt-Ti/C

Twenty grams of catalyst consisting of 10% Pt, by weight, supported oncarbon black was ultrasonically dispersed in 800 ml distilled water. Thesurface area of the platinum in the catalyst exceeded 110 m² /g Pt. In aseparate beaker one gram of finely divided TiO₂ (such as P-25manufactured by Degussa of Teterboro, N.J.) was dispersed in 400 mldistilled water. The two suspensions were mixed together and stirred tobring them into intimate contact. The mixed suspensions were caused tofloc by moderate heat. The solids were filtered off and dried providingan intimate mixture of TiO₂ and Pt/C catalyst. The mixture was heated to930° C. in flowing N₂ and held at this temperature for one hour. Theproduct was cooled to room temperature before exposing it to atmosphericair.

Electron microscopy and electrochemical measurements of severaldifferent batches made as indicated above gave specific surface areas ofgreater than 60 m² /g and as high as 80 m² /g of platinum in the alloy.X-ray diffraction analysis confirmed alloying in the form of Pt-Ti solidsolution.

The catalyst made as described was tested as the cathode catalyst inphosphoric acid fuel cells (98% H₃ PO₄ at 375° F.) and was found to havean activity for the reduction of oxygen at 0.9 V which was 90% higherthan that of the 10 Pt/90 C from which it was made (based on equivalentplatinum loadings).

EXAMPLE 2--Pt-Si/C

Several batches of a Pt-Si catalyst supported on carbon black wereprepared by essentially the same method as described in Example 1 exceptthat very finely divided SiO₂ (Aerosil-380 manufactured by Degussa) wassubstituted for the TiO₂. The co-suspension of SiO₂ and Pt/C floccedwithout heat. The filtered dried mixture was heated to 820° C. for onehour in nitrogen. The surface area of the metal in the product wasgreater than 60 m² /g and in certain batches greater than 85 m² /g ofplatinum in the alloy, and Pt-Si alloy formation was confirmed by X-raydiffraction. The supported alloy catalyst was fabricated into anelectrode and tested in a fuel cell. Its activity for the reduction ofoxygen in phosphoric acid was found to be 20% higher per mg of platinumthan that of the original Pt/C catalyst from which it was made.

EXAMPLE 3--Pt-Al/C

Degussa fumed Al₂ O₃ -C was used to prepare several batches of acatalyst using the method of Example 1. In this case a flocculatingagent, Al(NO₃)₃, was used to coat the carbon surface with Al₂ O₃particles and form a co-floc.

A metal surface area greater than 59 m² /g and in some instances greaterthan 75 m² /g of platinum in the alloy was measured and X-raydiffraction confirmed Pt-Al alloying. The fuel cell tests showed 110%activity improvement over the precursor.

EXAMPLE 4--Pt-Al-Si/C

Degussa fumed aluminum silicate (P-820) was used in the manner ofExample 1 to obtain high surface area Pt-Al-Si ternary alloy supportedon carbon black.

A metal surface area of 53 to 57 m² per gram of platinum in the catalystwas measured. The fuel cell tests showed about 30% increase in theactivity over the Pt/C catalyst.

EXAMPLE 5--Pt-Sr-Ti/C

Commercially available SrTiO₃ is not fine enough to be useful forpreparing a uniform high surface area Pt-Sr-Ti ternary alloy catalyst.However, reacting SrCO₃ with high surface area TiO₂ (Degussa--P25) atabout 1100° C. gave relatively high surface area SrTiO₃.

SrTiO₃ prepared as indicated above was used in the manner of Example 1to obtain high surface area Pt-Sr-Ti ternary alloy supported on carbonblack. A metal surface area of about 51 m² /g of platinum in thecatalyst was measured. The fuel cell tests showed about 20% increase inthe activity over the Pt/C catalyst.

EXAMPLE 6--Pt-Ce/C

Ten grams of catalyst consisting of 10% Pt by weight supported on carbonblack (the same catalyst used in Example 1) was ultrasonically dispersedin 700 ml distilled water. In a separate beaker 1.0 gram of ceriumammonium sulphate was dissolved in 50 ml distilled water. The two weremixed together and chilled to 0°-10° C. while constantly stirring. ThepH of this suspension was slowly increased to 5.0-6.0 using cold 1.0 NNaOH. It is believed that fine gelatinous hydrous ceric oxide, CeO₂.xH₂O so formed is immediately adsorbed on the available surface on thecarbon and does not floc in the precipitate form.

After such treatment the solids were filtered and dried providing highlydispersed CeO₂.xH₂ O and highly dispersed Pt co-supported on carbon. Themixture was sub-divided in three batches and heated to 700° C., 800° C.and 950° C., respectively. Surface areas of 64, 68.9 and 52.6 m² /g,respectively, of platinum in the catalyst were measured. X-raydiffraction analysis confirmed alloying. The fuel cell tests showed upto 40% increase in activity.

Although the examples given above are related to carbon supportedplatinum alloy catalysts for use in acid fuel cells, the method of theinvention and the advantages associated therewith are generallyapplicable to supported or unsupported noble metal alloys used in otherapplications such as catalysts for base fuel cells, chemical andpharmaceutical processes and automotive and anti-pollution devices.

If a finely divided unsupported alloy is to be made in accordance withthe present invention the starting materials would be a finely dividedunsupported noble metal (such as platinum black) and a finely dividedrefractory metal oxide. A co-dispersion of these particles is formed ina liquid such as water and the solids are thereafter separated from theliquid and dried. The dry solids are then heated in a reducingatmosphere to form the alloy in the same manner as described above.

Although the invention has been shown and described with respect toillustrative embodiments thereof, it should be understood by thoseskilled in the art that other changes, omissions and additions in theform and detail thereof may be made without departing from the spiritand scope of the invention.

Having thus described typical embodiments of our invention, that whichwe claim as new and desire to secure by Letters Patent of the UnitedStates is:
 1. A method for making carbon supported noblemetal-refractory metal alloy catalysts comprising:(a) providing asupported unalloyed noble metal catalyst in the form of a carbon supportmaterial having finely-divided noble metal particles uniformly dispersedon its surface; and (b) reacting the carbon supported noble metalcatalyst particles with a refractory metal oxide by intimatelycontacting the carbon supported noble metal particles with finelydivided particles of said metal oxide and heating to a sufficiently hightemperature in a locally reducing environment to thermocatalyticallyreduce the oxide and at the same time form an alloy between the noblemetal and the refractory metal, wherein said carbon support materialprovides the locally reducing environment and the support for saidalloy.
 2. The method of claim 1 wherein intimate contact between thesupported noble metal particles and refractory metal oxide is achievedby soaking the noble metal-covered carbon support material in acolloidal dispersion of the refractory metal oxide and then drying thecarbon support material after soaking to produce an intimate mixture ofsupported noble metal catalyst particles and metal oxide particles onthe carbon support material.
 3. The method of claim 1 wherein thecontacting noble metal particles and refractory metal oxide are heatedto at least about 600° C. to effect the reaction.
 4. The method of claim3 wherein the temperature is 800°-1000° C.
 5. The method of claim 1wherein the noble metal is platinum.
 6. The method of claim 1 whereinthe refractory metal is tungsten, aluminum, titanium, silicon, cerium,strontium, or combinations thereof.
 7. The method of claim 6 wherein thesurface area of the resulting alloy is at least 30 m² /g of noble metalin the alloy.